Pyrolysis of fluorocarbons to hexafluoropropylene



Jan. 31, 1961 E. H. TEN EYCK EIAL 2,970,176

- PYROLYSIS OF FLUOROCAREONS T0 HEXAFLUOROPROPYLENE Filed Oct. 31, 1957 INVENTORS GEORGE P. LARSON EDWARD H. TEN EYCK BY W ATTORNEY 2,970,176 PYRoLYsIs on LUoRocARBoNS ro- HEXAFLUOROPROPHENE Edward H. Ten Eyck, Parkersburg,-W.- Va;,; and George P. Larson, Elizabeth, N'.J.,. assignors-td E. I. du- Pont de Nemours'and Company, Wilmington, Deli, a. cor.- poratlon of Delaware Filed Oct. 31, 1957', Ser. No. 693,719.

SCIaims. (l.-260653 .3)'

tetrafiuoroethylene at 665 givesa 42% yield of hexa fluoropr'opylene at a contact time of to seconds, the yield of hexafiuoropropylene d'roppihgto 4% when. the temperature was raised to 750:5 "1 Recently it has been reported" that at 800 C, at atmospheric pressure, the yield of hexa fluoropropylene from tetr-afluoroethylene is only2'.l% (J.C.S. 1953, 2083). Thus inthese previously known procedures the yields of hexafluoropropylme were relatively small. It was believed that the cause of the low yields of hexafluoropropylene was the preferential formation of higher molecular weight fluorocarbon's which were sufliciently stable to resist further reaction. Very recently a pyrolysis process was discovered (US. Patent 2,758,138 issued to D; A. Nelson, August. 7, 1956) which gave rise to high yields of hexafluoropropylene from tetrafiuoroethylene under conditions such. that the formation of the higher molecular weight fluoroc'a'rbon's was" essentially avoided. This process was carried out at very low pressures with limited flow rates. Although this process gave rise to hexafluoropropylene' yields" of 75% on the basis of the tetrafluoroethylene fed it still caused a substantial lossof tetrafl'uoroethylene and was difficult to operate in that it required very low pressures.

It is one of the objectives of the present invention to provide a process for the preparation of hexafluoropropylene from tetrafiuoroethylene wherein an. essentially 100% yield to heit afluoropropylene is obtained; It is another object of the present invention to provide a process which can be operated at atmospheric pressure. A further object of the present invention is the pyrolysis of higher molecular weight gaseous fiuorocarbons to tetrafiuoroethylene and hexafiuoropropylene. Other objects. will become apparent hereinafter.

The objects of the present invention are accomplished by a process which comprises continuously feeding a mixture of tetrafiuoroethylene and higher boiling fluorocarbons into a reaction zone at a temperature of 700 to 900 C., separating the hexafluorop-ropylene formed, recycling the remaining fluorocarbon compounds and con.- tinuously adding a quantity of tetrafiuoroethyle'ne, which is equal to the quantity of tetrafluoroethylene. converted to hexafiuoropropylene, to the recycling fluorocarbon A United States Patent 9 lute) a mixture of tetrafluoroethylene. and fluorocarbons.

of. the class consisting of C F C F and C F wherein n is a whole number froml to 10, the molar ratio of saidfluorocarbons to tetrafluoroethylene being at least 0.050, cooling the reaction mixture, continuously separating. the hexafluoropropylene and recycling the remaining reaction products, and adding a quantity of tetrafluoroethylene which corresponds to the quantity of tetrafluoroethylene converted to hexafluoropropylene. Under these conditions it was found that all the tetrafluoroethylene reacting was converted to hexafluoropropylene so that an essentially yield of hexafiuoropropylene was obtained.

The process of the present invention is based on the discovery that the formation of. hexafluoropropyl'ene is not limited to the following reaction:

but that higher fluoroolefins and fluorocarbons are'capable of supplying difluoromethylene radicals which can form hexafluoropropylene or tetrafiuoroethylene under the conditions of the present invention. This is rather surprising since it was hereto-fore believed that the higher boiling fluorocarbons would continue to build up without leading to the formation of additional tetrafluoroethylene or hexafiuoropropylene and that, consequently, these higher fiuorocarbons' had to be removed from a continuous process. Thus the prior art processes never achieved an essentially 100% yield of hexafluo-ropropylene from tetrafluoroethylene reacted.

The composition of the fluorocarbon mixture employed in the present invention is not critical, except that the fluorocarbons which are employed should be gaseous at the temperature at which the reaction occurs. Since the formation of hexafiuoropropylene occurs through the formation of difluorom'eth-ylene radicals rather than through the formation of fluorine radicals, it is generally preferred to employ a fluorocarbon mixture which has a fluorine to carbon ratio of close to two and not exceeding three. Thus fluoroolefins having more than three carbon atoms per molecule and saturated fluorooarbons having more than two carbon atoms are generally preferred. A suitable higher fluorocarbon mixture may be obtained by passing tetrafluoroethylene through the reaction zone at the conditions outlined above, removing the hexafluoropropylene obtained, and recycling the remaining fluorocarbons. The fluorocarbons thus obtained comprise some saturated fluorocarbons having from 2 to 6 carbon atoms and besides tetrafiuoroethylene and rhexafluoropropylene, higher fiuoro" olefins having from 4 to 10 carbon atoms. Any other source of higher fiuorocarbons, such as obtained from the pyrolysis of polytetrafluoroethylene is, of course, equally suitable. As stated hereinabove, the ratio of the higher fluorocarbons to .the tetrafiuoroethylene fed should be at least 0.05. A preferred range oi ratios is from 0.075 to 1.0.

The process of the present invention is carried out at a temperature of 700 to 900" C. At temperatures above sures allow high feed rates. At low pressures, the amounti of starting material fed is significantly lower for the same size of equipment since a minimum contact time must be maintained, and it is furthermore more difficult to maintain a uniform heat transfer to the reactants fed. An additional disadvantage is the higher cost and maintenance of low pressure equipment. Pressures greatly' above atmospheric pressure are generally avoided for the sake of safety, since the reaction is exothermic in nature and may lead to serious explosions should excessive overheating take place; the danger of overheating increases with increasing pressure. A suitable pressure range is from 0.2 p.s.i. to 65 p.s.i. (absolute).

In order to obtain a complete conversion of tetrafluoroethylene reacting to hexafiuoropropylene at the temperatures employed in the process of the present invention, a minimum contact time of at least 0.05 .sec. is necessary. Longer contact times such as up to 2 sec. may be employed if desired.

A further significant factor that must be considered in carrying out the process of the present invention is the heat transfer of the gas in the reaction zone. Since the reaction employed in the process of the present inven tion is exothermic in nature insufficient heat transfer will cause local overheating and carbonization of the reaction products and thus detract from the conversion of the tetrafluoroethylene to hexafluoropropylene and cause plugging of the reaction unit which necessitates costly repair and idleness of the equipment. Overheating which occurs more frequently at higher pressures may be prevented by controlling the flow of the gas through the reaction unit. Flow of a gas through a pipe may generally be characterized by its Reynolds number although laminar flow, gen- I erally having Reynolds numbers below 2100, may suitably be employed at low pressures where overheating is less likely to occur, it is generally preferred to employ a turbulent flow having Reynolds numbers above 2500 to avoid overheating. When operating at atmospheric pressures, it is necessary to employ a gas flow having Reynolds numbers above 2500. Although the gas may be more evenly heated in a pipe containing an inert filler,

such are not preferred since it was found that filled tubes cause a significant pressure drop which is preferably avoided.

One method of operating the process of the present invention is further illustrated by the attached drawing schematically showing the process of the present invention. Tetrafluoroethylene is admitted to the process stream and admixed with recycled fluorocarbons either boiling above hexafiuoropropylene '11 or below hexafluoropropylene 12 in the desired proportion.

The 1 mixture is thenintroduced into the furnace 13 where" the tetrafluoroethylene is converted to hexafluoro'propyl- 4 17 where those fluorocarbons boiling below hexafluoropropylene particularly unreacted tetrafluoroethylene are distilled off and recycled 12. To prevent polymerization of the perfiuoroolefins in the reaction products, a polymerization stabilizer 18 is added at the top of the column. The hexafluoropropylene, higher boiling fluorocarbons and stabilizer are passed to a second column 19 where the hexafluoropropylene is distilled off and condensed in the refrigerated storage containers 20. The remaining fluorocarbons are passed into the hash tank 21 where the stabilizer is separated which may then be recycled to the primary column 17. The higher fluorocarbons separated from the stabilizer are recycled and admixed with tetrafiuoroethylene make-up.

If it is desirable,'the pyrolysis feed may be premixed .inthe storage tank 23.

'The invention is further illustrated by the following examples which were obtained by employing a reaction unit substantially as illustrated in the attached drawing.

Examples 1 to VII .-Into a down-flow reactor compris ing a 3 furnace containing a 15' long, /z" diameter stainless steel pipe arranged in loops, heated to the temperature indicated in column 3 of the table below was charged at a pressure indicated in column v4 of the table, a reaction mixture comprising tetrafiuoroethylene and higher boiling fluorocarbons at the rates indicated in colmm 2 and 8 of the table.

a cooling section comprising a water cooled 3 section /z" diameter pipe into an acid absorber bed of soda lime,

12 inch in diameter and 18" deep, to remove any hydro- The reaction, mixture was then passed into a compressor where the line,

fluoric acid formed during the reaction.

. and +10 C. at the bottom of the column by cooling with a brine solution. Terpene B, a commercially available terpene polymerization inhibitor consisting of a mixture of dipentene and terpinolene was introduced at the top of the column at the rate of 1.25 cc. per hour. Substantially pure tetrafluoroethylene was obtained at the,

top of the column and was combined with makeup tetrafluoroethylene and then recycled to the pyrolysis furnace.

The higher boiling fluorocarbons were removed from the bottom of the still passed through a pressure reduction valve into a second distillation column of identical construction as the first maintained at a pressure of p.s.i.g.l

and a temperature of +20 C. at the top of the column and +40 C. at the bottom of the column. Hexafluoro-' propylene was taken off at the top of the column and the higher boiling fluorocarbons were taken from the bottom of the column and combined with the tetrafluoroethylene.

and recycled to the pyrolysis furnace. The table below shows the Reynolds flow number in the furnace, the conversion of tetrafiuoroethylene (TFE) to hexafluoropro-- pylene (HFP), the yield of hexafiuoropropylene on the basis of the tetrafluoroethylene reacted and the quantity of higher fluorocarbons recycled. The higher fiuorocarbons recycled were analyzed to be mixtures of cornpounds having the following compositions; C F C F 4Fs s s, 05PM, CGF'D CGFS: s iol 7 7; C7F8 'z n, C F C F The analysis of the higher boiling fluoro-'- carbons was carried out by mass spectroscopy and gas chromatography.

In the table below Example 1 demonstrates the results obtained in the absence of any higher boiling fluorocarbons. Examples 3 and 4 demonstrate the ability of higher fluorocarbons to form hexafluoropropylene. Ex-' amples 2, 5, 6 and 7 demonstrate substantially 75 of the additional tetrafiuoroethylene fed.

The contact time of the gaseous reaction mixture in the reactor was 0.5 sec." The reaction mixture was passed from the reactor through Conver- Percent High 'IFE Reaction Furnace sion to HFP B iling Example N0. Feed, Temp., Pressure Reynolds HFP, Yield Flunrop.p.h. C. in H2O, N0. Mole from carbons Gage Percent TFE Recycled,

p.p.h.

Hexafluoropropylene obtained by the process of the present invention is highly useful as a monomer in the preparation of fluorinated polymers and particularly in the preparation of copolymers with such comonomers as tetrafluoroethylene, vinyl fluoride, vinyl chloride, methyl methacrylate and the like. The polymers thus obtained are useful for molding and extruding into fibers, films and other shapes, including the manufacture of articles which are of value in electrical insulation.

By the process of the present invention tetrafluoroethylene and other high boiling fluorocarbons can be quantitatively converted to hexafiuoropropylene and thus the present invention provides a highly economic method for preparing hexafluoropropylene. The process is further advantageous in that it can be operated at atmospheric pressure.

We claim:

1. A process for preparing hexafiuoropropylene which comprises continuously feeding into a reaction zone maintained at a temperature of 700 to 900 C. and at a pressure of 0.2 p.s.i. to 65 psi for a contact time.

of at least 0.05 see. a mixture of tetrafluoroethylene and higher boiling fluorocarbons of the class consisting of a mixture Of C F C F2 +3 and C F wherein n is a number from 1 to 10, the molar ratio of said higher boiling fluorocarbons to said tetrafluoroethylene being at least 0.05, cooling said reaction products, separating hexafluoropropylene therefrom, recycling the remaining fluorocarbons and continuously adding tetrafluoroethylene to said recycling fluorocarbons in the quantity of tetrafluoroethylene converted to hexafluoropropylene without substantially altering the amount of said higher boiling fluorocarbons.

2. A process for preparing hexafiuoropropylene from tetrafluoroethylene which comprises continuously feeding into a reaction zone maintained at a temperature of 700 C. to 900 C. and at a pressure of 0.2 p.s.i. to

p.s.i. for a contact time of at least 0.05 second under How conditions exceeding a Reynolds number of 2500, a mixture of tetrafluoroethylene and higher boiling fluorocarbons of the class consisting of a mixture of C F Cn+3F2(n+3) and Cn+3F2(n+2) wherein n is a number from 1 to 10, the molar ratio of said higher boiling fiuorocarbons to said tetrafluoroethylene being at least 0.05, cooling said reaction products, separating hexafiuoropropylene therefrom, recycling the remaining fluorocarbons and continuously adding tetrafluoroethylene to said recycling fluorocarbons in the quantity of tetrafluoroethylene converted to hexafluoropropylene without substantially altering the amount of said higher boiling fluorocarbons.

3. A process for preparing hexafluoropropylene which comprises continuously feeding into a reaction zone maintained at a temperature of 700 to 900 C. and at atmospheric pressure for a contact time of at least 0.05 sec. under flow conditions exceeding a Reynolds number of 2500, a mixture of tetrafluoroethylene and higher boiling fluorocarbons of the class consisting of a mixture of C l- Cn+3F2(n+3) and C F wherein n is a whole number from 1 to 10, the molar ratio of said higher boiling fiuorocarbons to said tetrafluoroethylene being at least 0.05, cooling said reaction products, separating hexafluoropropylene and recycling the remaining fluorocarbons and continuously adding tetrafluoroethylene to said recycling fiuorocarbons in the quantity of tetrafluoroethylene converted to hexafiuoropropylene with out substantially altering the amount of said higher boiling fluorocarbons.

References Cited in the file of this patent UNITED STATES PATENTS 2,442,324 Heitz et al. May 25, 1948 2,758,138 Nelson Aug. 7, 1956 2,759,983 Waddcll Aug. 21. 

1. A PROCESS FOR PREPARING HEXAFLUOROPROPYLENE WHICH COMPRISES CONTINUOUSLY FEEDING INTO A REACTION ZONE MAINTAINED AT A TEMPERATURE OF 700 TO 900* C. AND AT A PRESSURE TO 0.2 P.S.I. TO 65 P.S.I. FOR A CONTACT TIME OF AT LEAST 0.05 SEC. A MIXTURE OF TETRAFLUOROETHYLENE AND HIGHER BOILING FLUOROCARBONS OF THE CLASS CONSISTING OF A MIXTURE OF CNR2N+2 CN+F2(N+3) AND CN+3F2(N+2) WHEREIN N IS A NUMBER FROM 1 TO 10, THE MOLAR RATIO OF SAID HIGHER BOILING FLUOROCARBONS TO SAID TETRAFLUOROETHYLENE BEING AT LEAST 0.05, COOLING SAID REACTION PRODUCTS, SEPARATING HEXAFLUOROPROPYLENE THEREFROM, RECYCLING THE REMAINING FLUOROCARBONS AND CONTINUOUSLY ADDING TETRAFLUOROETHYLENE TO SAID RECYCLING FLUOROCARBONS IN THE QUANTITY OF TETRAFLUOROETHYLENE CONVERTED TO HEXAFLUOROPROPYLENE WITHOUT SUBSTANTIALLY ALTERING THE AMOUNT OF SAID HIGHER BOILING FLUOROCARBONS. 